This invention relates to the controlling of the operational steps of processes such as tire vulcanization by use of an input program means. More specifically, this invention relates to a method which totally controls the vulcanizing process either in terms of steps having a time duration or in terms of steps having a duration proportional to cure equivalents.
Many industries utilize machinery which performs sequential steps of varying lengths on a workpiece. The vulcanization of a tire in a tire curing press, for example, is such a process having numerous steps of varying duration. Precise control of these steps is quite important not only to the quality of the finished product but also for the efficient use of the particular machinery. While varying degrees of sophistication exist in devices to control the various operations as a function of time, certain deficiencies nevertheless exist in all controls.
For example, many prior art devices which utilize a "program" type of input require a great deal of manual "set up" on the part of an operator. One such device utilizes mechanical stepping switches which must be set at the desired location along a shaft or the like so that they are tripped at the desired time. Even those devices which utilize a card type input are cumbersome in that special cards designed for the specific use are employed rather than the more standard data entry means available.
Because the tire vulcanization process, like other manufacturing processes, is made up of sequential steps having significantly variable lengths, the program devices of the prior art all are forced to provide capabilities for each step dependent on the longest step to be encountered. This practice, of course, proves to be quite wasteful in that much program space and the attendant electronic circuitry is not at all necessary for the majority of the operating cycle.
Other supposedly automatic control systems utilize complex mechanical or electromechanical devices which are expensive to manufacture and not always reliably accurate. Many attempts at providing a total electronic system have utilized analog circuitry which does not provide a satisfactory degree of accuracy.
Further, present systems utilized for controlling the process of tire vulcanization are capable of satisfactory operation only in a time mode. The rubber industry has developed a standard known as a "cure equivalent" which may be defined as one minute of curing time at a constant reference temperature, typically about 300.degree.F. Thus, it may be desirable for a particular tire compound to be cured for twenty cure equivalents, which, in the long run, is a much more accurate parameter than pure time duration because the actual temperature of the rubber is being monitored.
Certain prior art work has been done in the area of analog calculation of cure equivalents for various purposes, but no presently marketable device utilizes such calculations to control stepped press operations. One problem in calculating the cure equivalent factor based on the Arrhenius function involves the necessary means of obtaining true temperatures internally of the tire, these temperatures being used as inputs to the cure equivalent calculation circuitry. While such has been done experimentally through the use of thermocouples embedded in the tire, no system has been devised which will monitor and operate the standard "twin" vulcanizing press. In this press, two tires are cured at one time. This is no problem for those prior art devices which control press operations as a fuction of time, but has apparently been a problem which has prevented prior art devices from utilizing cure equivalents as a control standard since one tire in a "twin" press may be curing faster than the other tire. No device has been developed which will satisfactorily account for the possible variances between the temperatures of two tires within a twin press.
Further, by using thermocouples to read the temperature, it is necessary to provide sometimes complex electronic circuitry which will convert the temperature signals to one which is proportional to the Arrhenius function. Despite this problem, no satisfactory substitute for the thermocouple input has been proposed.